FDM & OFDM
In Radio Access (FDMA) systems, separate radio frequencies with specific deviations are allocate for broadcast and reception. For example, in the GSM system, 900 MHz (890 - 915 MHz for the uplink direction, 935 - 960 MHz for the downlink direction), 124 separate wavelengths or radio channels with a specific frequency of 200 MHz are allocate for the base transceiver. Station) and Mobile Station (Mobile Station) / User Equipment (User Equipment) to avoid radio interference between each other.
In OFDM radio access systems, several subcarrier conductors are place close to each other but need to have orthogonality between them to avoid interference.
LTE systems the subcarrier wavelength has a constant value of 15 kHz, whereas in 5G NR the scalable subcarrier spacing is used for appropriate use with appropriate frequency. 15 × 2 n kHz (n = 1 corresponds to a gap of 30 kHz, n = 2 corresponds to 30 kHz… etc.)